The largest coral reef system in the world—and the biggest sign of life on Earth, visible from space—is not growing like it used to. A sampling of 328 massive Porites coral (large structures resembling brains that are formed by tiny polyps) from across the 133,000-square-mile (344,000-square-kilometer) reef reveals that growth of these colonies has slowed by roughly 13 percent since 1990.

The most likely reason is climate change caused by increasing carbon dioxide (CO2) and other greenhouse gases in the atmosphere, according to a new paper published today in Science.

The burning of fossil fuels over the past century or so has driven atmospheric CO2 levels from 280 parts per million (ppm) to 387 ppm—and growing. More than 25 percent of this extra CO2 is absorbed by the world's oceans and reacts with seawater to form carbonic acid. A rising carbonic acid level means a more acidic ocean.

And a more acidic ocean is bad news for coral and other sea creatures, which form their shells from calcium carbonate they extract from seawater. The more acidic the water, the more difficult it is to build the shells in the first place—as well as keeping them from dissolving.

To probe how corals are faring, marine biologist Glenn De'ath and colleagues at the Australian Institute of Marine Science in Townsville, Queensland, examined Porites coral samples stretching as far back as 1572. Because Porites lay down annual layers—like tree rings—changing environmental conditions are etched into their skeletons.

The record has not been good in recent years: Since 1990 coral have been extending and thickening by less and less each year. "The data suggest that such a severe and sudden decline in calcification is unprecedented in at least the past 400 years," the researchers wrote.

"This study put all this worry and discussion [about ocean acidification] into a real-world context," says marine biologist John Bruno of the University of North Carolina at Chapel Hill. "It shows that coral growth is indeed slowing—over a huge range and at many reefs—potentially due to increased acidity."

Slower growth will mean both that existing coral will find it difficult to cope with escalating acidity and rising sea levels. This will have enormous knock-on effects in sea life that relies on coral reefs for habitat—as well as human fisheries and other ecosystem services.

In the meantime, it appears that changes in sea temperatures and increased acidity are already beginning to impact the Great Barrier Reef. "Our data show that growth and calcification of massive Porites in the Great Barrier Reef are already declining and are doing so at a rate unprecedented in coral records reaching back 400 years," the researchers wrote. "These organisms are central to the formation and function of ecosystems and food webs, and precipitous changes in the biodiversity and productivity of the world's oceans may be imminent."